Front seat in bus allocation rules

I was recently going from Kandy to Hatton in a bus and I was seated with my grandma who have trouble walking so she uses a stick. Somewhere around gampola 1 young monk who'll probably be somewhere between 16 to 18 got on the bus and the people behind us asked us to stand . I stood up since he was the only one and a guy behind me yelled asking my grandma who can't walk without any help to stand up . So she stood up even though I asked her not to with her leg issues. Noone even offered her a seat, we got down in the next stop. I'm completely fine with giving them a seat to sit but there was only 1 monk , why can't they share their seat? Even if it's one person . They need to have both the seats? I don't knowif I'm wrong somewhere but if I'm correct me.